Add Codex Chat-to-Responses bridge

- Add Codex provider API format selection and model mapping for Chat-only upstreams.
- Convert Codex Responses requests to Chat Completions and rebuild Chat responses as Responses output.
- Preserve reasoning_content across non-streaming, streaming, tool calls, and previous_response_id follow-ups.
- Add a bounded Codex Chat history cache for restoring tool calls before tool outputs.
- Cover Chat bridge, compact routing, streaming, and history recovery with focused tests.

src/i18n/locales/en.json

@@ -1080,6 +1080,8 @@
     "saveFailed": "Save failed: {{error}}",
     "modelNameHint": "Specify the model to use, will be auto-updated in config.toml",
     "modelName": "Model Name",
+    "upstreamModelName": "Upstream Model Name",
+    "upstreamModelNameHint": "For Chat format, enter the real upstream model here; takeover writes a Codex-compatible local model and routing maps it back to this model.",
     "modelNamePlaceholder": "e.g., gpt-5-codex",
     "contextWindow1M": "1M Context Window",
     "autoCompactLimit": "Auto Compact Limit",

src/i18n/locales/ja.json

@@ -1080,6 +1080,8 @@
     "saveFailed": "保存に失敗しました: {{error}}",
     "modelNameHint": "使用するモデルを指定します。config.toml に自動更新されます",
     "modelName": "モデル名",
+    "upstreamModelName": "上流モデル名",
+    "upstreamModelNameHint": "Chat 形式ではここに実際の上流モデルを入力します。接管時は Codex 互換のローカルモデルを書き込み、ルーティングでこのモデルに戻します。",
     "modelNamePlaceholder": "例: gpt-5-codex",
     "contextWindow1M": "1M コンテキストウィンドウ",
     "autoCompactLimit": "自動圧縮しきい値",

src/i18n/locales/zh.json

@@ -1080,6 +1080,8 @@
     "saveFailed": "保存失败: {{error}}",
     "modelNameHint": "指定使用的模型，将自动更新到 config.toml 中",
     "modelName": "模型名称",
+    "upstreamModelName": "上游模型名称",
+    "upstreamModelNameHint": "Chat 格式下这里填写真实上游模型；接管时 Codex 本地会使用兼容模型，并由路由映射回该模型。",
     "modelNamePlaceholder": "例如: gpt-5-codex",
     "contextWindow1M": "1M 上下文窗口",
     "autoCompactLimit": "压缩阈值",